Conventionally, there is a vehicle including a pedestrian protective device for reducing an impact on a pedestrian when the pedestrian collides with the vehicle. This vehicle is provided with a collision detection device having a sensor at a bumper portion. If it is detected by this sensor that a pedestrian or the like collides with the vehicle, the pedestrian protective device is activated to mitigate an impact on the pedestrian. For this pedestrian protective device, there is a device called a pop-up hood, for example. At the time of detection of the collision of the vehicle, this pop-up hood lifts up a rear end of an engine hood to increase a distance (clearance) between the pedestrian and a hard component such as an engine. The pop-up hood absorbs the energy of the collision with the pedestrian's head by using this space to reduce the impact on the head.
As the above-described collision detection device for a vehicle, there is a device that includes a chamber member having therein a hollow portion on a front surface of a bumper reinforcement in the vehicle bumper and that detects the pressure in the hollow portion of this chamber member by a pressure sensor. In the device having this configuration, when an object such as a pedestrian collides with the bumper (bumper cover), the chamber member is deformed due to the deformation of the bumper cover to cause a pressure change in the hollow portion of the chamber member. The device detects the collision of the object through the detection of this pressure change by the pressure sensor (see, for example, JP2009-23407A, U.S. Patent Application Publication 2009/0024323).
In the collision detection device for a vehicle having the above configuration, a clearance may be made because contour shapes such as curvature differ between a rear surface of the chamber member and the front surface of the bumper reinforcement. In this case, at the time of the collision between the vehicle and the pedestrian or the like, the clearance existing on a vehicle-rear side of the chamber member influences the deformation of the chamber member in accordance with the deformation of the bumper cover. Accordingly, there is an issue that the accuracy (time-responsiveness) in detection of the pressure change by the pressure sensor is reduced and that the accuracy in the collision detection may thereby be decreased.